This application is based on application Nos. 98-50653 and 99-5564 filed in the Korean Industrial Property Office on Nov. 25, 1998 and Feb. 10, 1999, the content of which is incorporated hereinto by reference.
(a) Field of the Invention
The present invention relates to a negative active material for a lithium secondary battery, a method of preparing the same and a lithium secondary battery comprising the same and, more particularly, to a negative active material for a lithium secondary battery having a high discharge capacity as well as high charge and discharge efficiencies.
(b) Description of the Related Art
Metallic lithium was used in the past for negative active material in a lithium secondary battery. However, lithium metal has good reactivity toward electrolyte and deposits to form a dendric layer which increases the reactivity of lithium. There are at least two disadvantageous effects that arise from the reaction of lithium with electrolyte: the exothermic liberation of heat and the formation of a passive film on the negative electrode. The formation of passive films on lithium is recognized as one of the reasons for the loss of capacity of lithium cells on repeated cycling.
The problem of lithium reactivity toward the electrolyte is addressed by replacing lithium metal with carbon-based materials. With the use of carbon-based active materials, the potential safety problem present in metallic lithium-based batteries can be prevented while achieving a relatively higher energy density as well as a reasonable shelf life. However, carbon-based material exhibits a relatively lower discharge capacity, and charge and discharge efficiency.
The choice of electrolyte solvents is critical to the battery performance. When propylene carbonate is employed for the electrolyte solvent, the graphite-based negative active material is prone to excessively react with propylene carbonate. Therefore, the available candidates of electrolyte solvents for ensuring a relatively stable reaction are ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate. Among these materials, ethylene carbonate is usually recommended for use as the electrolyte solvent as it ensures a reasonable cycle life of the battery. However, ethylene carbonate has a disadvantage in that as its content increases, the freezing point of the electrolyte is elevated. This is due to the fact that ethylene carbonate has a high melting point exceeding the ambient temperature.
In order to solve such problems, ethylene carbonate is replaced by propylene carbonate. However, propylene carbonate excessively reacts with the graphite-based active material.
It is an object of the present invention to provide a negative active material for a lithium secondary battery having a high discharge capacity as well as high charge and discharge efficiencies.
It is another object of the present invention to provide a method of preparing the negative active material for a lithium secondary battery.
It is still another object of the present invention to provide a lithium secondary battery including the negative active material.
In order to achieve these objects, the present invention provides a negative active material for a lithium secondary battery including a crystalline graphite core and a carbon shell formed around the core. The carbon shell includes at least one material selected from a transition metal, an alkali metal, an alkaline earth metal, an element of Group 3B of the Periodic table, an element of Group 4B, an element of Group 5B or a mixture thereof. The carbon shell is a turbostratic carbon layer, or an amorphous or crystalline carbon layer having different physical properties from the core.
The present invention provides a method of preparing a negative active material for a lithium secondary battery. In the method, an additive material is dissolved in water or an organic solvent, and the additive solution is mixed with a carbonaceous material. The additive material includes at least one material selected from a transition metal, an alkali metal, an alkaline earth metal, an element of Group 3B of the Periodic table, an element of Group 4B, an element of Group 5B or a mixture thereof. The carbonaceous material is natural graphite, artificial graphite, cokes, soft carbon, hard carbon or a mixture thereof. The mixture is dried, thereby doping the additive material on the surface of the carbonaceous material, and then heat-treated. The drying step of the mixture may be a spray-drying step.
The present invention further provides a lithium secondary battery including a negative electrode with the negative active material. The lithium secondary battery includes a positive electrode with a transition metal oxide-based active material. The separator is interposed between the negative and positive electrodes. The positive and negative electrodes and the separator are immersed in an electrolyte. The electrolyte includes propylene carbonate or ethylene carbonate and a lithium salt.